1. Field of the Invention
This invention relates to methods and apparatus for improving the performance of electronic devices, and more specifically to methods and apparatus using cross-degeneration techniques for improving the linearity, noise, gain and power consumption characteristics of transconductance cells, and of amplifiers, mixers, continuous-time filters and other active elements based upon such cells.
2. Description of Related Art
As is well known, linear amplification is a very important function in many electronic devices including those designed for use in signal processing applications. Under ideal conditions, the transconductance, gm, of an amplifying circuit remains constant for all input values. Disadvantageously, practical amplifiers are typically implemented with devices that are fundamentally non-linear. As a consequence, the transfer functions of the non-linear devices vary greatly and depend upon the magnitude of the applied input signal. The prior art is replete with attempts at improving the performance characteristics of electronic devices using these non-linear components. Prior art circuit design techniques have been proposed that compensate for variations in the transfer functions of non-linear devices and thereby increase the input range of circuits using these devices, without also adversely affecting noise characteristics. In amplification applications, for example, prior art circuit design techniques have been proposed that compensate for these non-linearities in order to maximize the magnitude of the input signal to be amplified.
In one exemplary prior art circuit design technique, the supply current is increased in order to increase the linear range overall gain of the circuit. Using another design technique commonly referred to as xe2x80x9cdegenerationxe2x80x9d, feedback is introduced into the design of the electronic circuit in an attempt to force the overall gain of the circuit to behave in a more linear manner. Disadvantageously, this approach substantially increases noise and power consumption while reducing gain and therefore may not be attractive in applications where noise or power considerations play an important role. Another technique combines signals produced by a plurality of interconnected devices in an attempt to more precisely shape and configure the overall device transfer function to meet specific design parameters. Disadvantageously, this technique requires use of an increased number of interconnected devices to proportionally increase the input range of the device. The performance of devices made in accordance with this technique disadvantageously depends upon the precision to which the devices are matched and upon the tolerance values of each circuit element. In addition, because an increased number of devices are needed, the technique also suffers from the disadvantages of increased overall circuit area, increased circuit complexity, increased power consumption, and reduced speed. Further, the applicability of this particular prior art technique is restricted to use in bipolar device technologies and sub-threshold MOS technologies.
FIG. 1 illustrates a circuit design technique referred to as a xe2x80x9chybrid doubletxe2x80x9d (HD) technique that combines degeneration with a multi-tanh signal shaping approach. A traditional Hybrid Doublet circuit 10 is shown in FIG. 1. The multi-tanh concept is a well known technique for extending the capacity of a transconductance gm cell (implemented in bipolar technologies), or an amplifier, mixer, continuous-time filter, or other active element based on such a cell, by using at least two differential transistor pairs operating in parallel. Each of the differential transistor pairs has a base offset voltage that splits the individual gm functions along the input-voltage axis. This allows the cell to process larger input voltage swings while allowing the overall transconductance of the cell to be more linear. The HD technique, and the description of the circuit 10 of FIG. 1, is described in more detail in a paper authored by Barrie Gilbert, entitled xe2x80x9cThe Multi-tanh Principle: A Tutorial Overviewxe2x80x9d, published in the IEEE Journal of Solid-State Circuits, Volume 33, No. 1, in January, 1998, which is incorporated herein by reference for its teachings on HD circuit design techniques.
The HD approach exemplified in the circuit of FIG. 1 suffers from many shortcomings. For example, the HD technique does not provide for high-order derivative cancellation. As a consequence, distortion cannot be carefully controlled or reduced. Another consequence of this limitation is that third-order distortion in the HD topology does not decrease monotonically with decreasing input power. In addition, as noted above in connection with the other prior art signal shaping approaches, the HD technique relies upon a precise matching of component values given a fixed power consumption specification. This restriction fixes the gain and the input range of the circuit. This, in turn, limits a circuit designer""s flexibility in tailoring the circuit for a desired application. For example, a circuit designer may wish to improve the input range and linearity of a circuit by allowing the circuit to consume more power. Disadvantageously, when selecting a topology based upon the prior art HD technique, the designer does not have this tradeoff flexibility. Thus, the components used in circuits designed in accordance with the HD approach are not easily scaleable.
Another disadvantage of the prior art HD approach is that circuit performance is quite sensitive to circuit parameters. As a consequence, variations in integrated circuit processes yield substantial degradations in signal distortion. A further disadvantage of circuits designed in accordance with the HD approach is that degeneration resistors (i.e., Re1 25 and Re1/Ae 50 of FIG. 1) result in a loss in current xe2x80x9cheadroomxe2x80x9d in the circuit. In addition, the choice of degeneration resistor values relies upon complex empirical mathematical formulae derived from the tanh characteristics of the hybrid doublet. Disadvantageously, this significantly constrains the flexibility and usefulness of the circuit. Consequently, circuit performance tradeoffs cannot be readily designed using the prior art HD approach. Because arbitrary input signal ranges cannot be accommodated, practical circuit performance parameters such as noise, gain, and linearity are not easily implemented using the HD approach. Finally, the HD approach disadvantageously is restricted to use in bipolar technologies.
Another exemplary design technique used in the prior art in an attempt to xe2x80x9clinearizexe2x80x9d electronic circuits utilizes sophisticated feedback circuits having high gain characteristics. An example of such a feedback circuit is an operational amplifier feedback circuit. However, similar to the other prior art approaches described above, this approach disadvantageously substantially increases the complexity of circuit design, reduces the overall speed and bandwidth of the device, increases the circuit area required to implement the device, and does so with limited enhancement of device linearity.
Therefore, the need exists for a method and apparatus that improves the performance of electronic devices, and more specifically, that enhances the linearity of electronic devices. The need exists for a compact circuit topology that provides enhanced device linearity as compared with traditional approaches. The enhancement in linearity should be accomplished: (a) without unduly increasing noise generated by a device; (b) without substantially increasing device complexity; (c) without substantially reducing the speed or bandwidth of the device; and (d) without substantially increasing the device power consumption. The power consumed by a device using this technique should be comparable to devices using the prior art xe2x80x9clinearizationxe2x80x9d approaches.
The method and apparatus for enhancing the linearity of electronic devices also should be easily xe2x80x9cscaleable.xe2x80x9d That is, the circuit should be able to accommodate a wide range of circuit specifications for linearity (or input signal range), noise, gain and power consumption, without requiring modifications to its topology (e.g., modification to the number of circuit devices or their interconnection). Furthermore, the inventive method and apparatus should find application in a wide range of device technologies, including bulk CMOS processes, and in circuits having three or more current legs (or to other three-terminal (or multi-terminal transconductance) device technologies such as MOS transistors). Further, a need exists for a method and apparatus that enhances the linearity of devices and achieves the above-stated objects, yet consumes a small percentage of integrated circuit area as compared with the entire device. The present invention provides such a method and apparatus using cross-degeneration techniques. The present invention improves the linearity, noise, gain and power consumption characteristics of transconductance cells, and of amplifiers, mixers, continuous-time filters and other active elements based upon such cells. In addition, the present invention facilitates the design of circuits that are robust with respect to drifts in component values caused by process or temperature variations.
The cross-degeneration method and apparatus of the present invention improves the linearity, noise, gain and power consumption characteristics of transconductance cells, and of amplifiers, mixers, continuous-time filters and other active elements based upon such cells. The present invention provides a plurality of circuit topologies wherein cross-degeneration resistors introduce cross-degeneration between current legs of an amplifying circuit. In one exemplary embodiment, the cross-degeneration resistors provide feedback between the current legs of transistor pairs (i.e., the current provided at the emitter outputs of a first transistor pair and the current provided at the emitter outputs of a second transistor pair).
The degeneration resistors dictate the total differential transconductance Gm of the inventive amplifying circuits. The differential transconductance Gm of the amplifying circuits are decreased by increasing the resistance values of the degeneration resistors; the differential transconductance are increased by decreasing the resistance values of the degeneration resistors. The degeneration resistors establish the flatness or variation in the total transconductance Gm as a function of the input voltage. The degeneration resistors set the flatness in the total transconductance by increasing the current diverted to an offset transistor pair. Increasing the resistance values of the degeneration resistors causes a corresponding increase in the curvature of the transconductance Gm at a zero input voltage swing.
In one exemplary embodiment, the inventive circuit topology includes a pair of offset resistors. The offset resistors establish an offset between the main current and an offset current. Increasing the offset resistance decreases the curvature of the transconductance Gm of the circuit at zero input swing. The third derivative of the transconductance Gm can be controlled, with respect to the input voltage swing, by similarly increasing (or decreasing) the resistance values of both of the degeneration resistors and the offset resistors. The present inventive circuit uses the cross-degeneration resistors to produce a multiple-sloped transfer function for each of the current branches of the exemplary circuits. The present invention uses the cross-degenerated resistors to linearize the inventive circuit instead of relying on exponential transfer characteristics such as the tanh or degenerated-tanh transfer functions. A linear transfer function can be obtained by summing the multiple-slope transfer function of each current branch of the exemplary inventive circuits.
A xe2x80x9ccommon-modexe2x80x9d variant of the present inventive cross-degeneration method and apparatus is provided. A motivation for the common-mode circuit topology is a desire to significantly reduce noise produced by circuit current sources. Noise attributable to the current sources is reduced by coupling the current sources to a common-mode point of the circuit. Consequently, noise produced by the current sources appears as common-mode signals only, and not as differential signals. The common-mode variant circuits of the present invention exhibit similar scalability and distortion tailoring functions as the basic cross-degeneration circuit.
The inventive topology provides a flexible, continuously-adjustable choice of input voltage ranges and gains given other constraints such as current consumption and noise. Using the present inventive method and apparatus, a wide range of input voltages and gains can be accommodated simply by varying component values, and without increasing circuit complexity or topology. Advantageously, this flexibility allows for straightforward design tradeoffs.
The details of the preferred and alternative embodiments of the present invention are set forth in the accompanying drawings and the description below. Once the details of the invention are known, numerous additional innovations and changes will become obvious to one skilled in the art.